Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+– Cl–, OH–, CO32–, SO42–, and PO43– aqueous systems (IUPAC Technical Report)

Abstract

This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl^–, OH^–, CO₃^2–, SO₄^2–, and PO₄^3–. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 °C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range I_c < 3.0 mol dm^–3.

Recommended values of log₁₀β_p,q,r° and the associated reaction enthalpies, ∆_rH_m°, valid at I_m = 0 mol kg^–1 and 25 °C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log₁₀β_p,q,r° values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H⁺ with CO₃^2– and PO₄^3–.

Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl₂(aq), Hg(OH)Cl(aq), and Hg(OH)₂(aq).